Mobile vehicle gear unit

ABSTRACT

A method of stepping down the speed of a rotary motion from a first speed, supplied to an input shaft, to a second speed delivered by an output shaft, comprises transmitting said rotary motion to said output shaft, via a driven gear of said output shaft, the driven gear being helical and having a first helix angle, from a layshaft arrangement, so as to generate a first axial thrust of the output shaft in a first axial direction; transmitting said rotary motion to said layshaft arrangement, via a drive gear of said input shaft, said drive gear being helical and having a second helix angle that is larger than said first helix angle, from said input shaft, so as to generate a second axial thrust of the input shaft in a second direction, said second direction being substantially opposite to said first direction; and applying said first axial thrust and said second axial thrust to the same location of an axially rigid support structure, such that said first and second axial thrusts counter-act and at least partly cancel out in said support structure.

FIELD OF THE INVENTION

The present invention relates to a method of stepping down the speed ofa rotary motion from a first speed, supplied to an input shaft, to asecond speed delivered by an output shaft. The present invention furtherrelates to a mobile vehicle gear unit comprising an input shaft, and anoutput shaft substantially parallel with said input shaft, the gear unitbeing configured for providing a non-unity transmission ratio betweensaid input shaft and said output shaft via a layshaft arrangement.

BACKGROUND OF THE INVENTION

The awareness and concern of the public with respect to environmentalimpact of transports is rapidly increasing. To meet such concern andawareness electric drive vehicles and low fuel-consumption vehicles aredesigned. An important aspect of an electric drive vehicle, and of alow-fuel consumption vehicle, is that the weight of the vehicle shouldbe low, to reduce consumption of power and fuel, respectively. Since themotor typically rotates at a relatively high rpm, whether it is anelectric motor or a petrol, diesel, biogas, or ethanol driven engine, agear unit is needed for stepping down the rotation speed of the motor orengine to a speed suitable for rotating drive wheels of the vehicle.

U.S. Pat. No. 4,297,906 discloses a gear box for a vehicle with asomewhat reduced weight. The gear box of U.S. Pat. No. 4,297,906 is,however, still too heavy for efficient use in electric vehicles and lowfuel-consumption vehicles, in particular when designed for therelatively high torques typical of electric motors.

There is a need for a gear unit offering higher reliability, lowerweight, and/or lower cost of manufacture.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve, or at least mitigate,parts or all of the above mentioned problems. To this end, there isprovided a method of stepping down the speed of a rotary motion from afirst speed, supplied to an input shaft, to a second speed delivered byan output shaft, the method comprising

transmitting said rotary motion to said output shaft, via a driven gearof said output shaft, the driven gear being helical and having a firsthelix angle, from a layshaft arrangement, so as to generate a firstaxial thrust of the output shaft in a first axial direction;

transmitting said rotary motion to said layshaft arrangement, via adrive gear of said input shaft, said drive gear being helical and havinga second helix angle that is larger than said first helix angle, fromsaid input shaft, so as to generate a second axial thrust of the inputshaft in a second direction, said second direction being substantiallyopposite to said first direction; and

applying at least a portion of said first axial thrust and at least aportion of said second axial thrust to the same location of an axiallyrigid support structure, such that said first and second axial thrustscounter-act and at least partly cancel out in said support structure.

The difference in helix angles will operate so as to make the magnitudesof said thrusts more equal, so as to increase the extent to which saidaxial thrusts at least partly cancel each other out. The resultant forceon the axially rigid support structure will thereby be limited. And byapplying said axial thrusts to the same location of said supportstructure, axial movement of the input and output shafts due to varyingload conditions can be minimized. As a comparison, in step-down methodsof prior art, input and output shafts generally apply their respectivedynamic axial thrusts onto opposite end walls of a gear unit housing,such that the gear unit housing flexes or yields somewhat under theaxial load, and the input and output shafts are thereby somewhat pushedapart. Such movement may lead to premature failure of bearings,particularly under varying load conditions. Axial movement also requiresa somewhat forgiving gear unit design, where substantive tooth clearanceor backlash allows some freedom of movement. By instead applying theoppositely directed axial thrusts to the same location of a supportstructure, it is possible to obtain an axially rigid relation betweenthe input and output shafts, such that the input and output shafts willnot move significantly in the axial direction relative to each other.Hence, a gear unit with tighter backlash, and thereby increasedlifetime, may be designed for such a step-down method. Throughout thisdisclosure, “substantially opposite directions” is to be construed assaid directions forming an angle of more than 165°.

According to an embodiment, said axially rigid support structure is amain thrust bearing arrangement interconnecting said output and inputshafts, such that said axial thrusts at least partly cancel out via themain thrust bearing arrangement. The main thrust bearing arrangement maycomprise one or several thrust bearings arranged as a “push bearingarrangement”, i.e. for providing support when the input and outputshafts are pushed towards each other; as a “pull bearing arrangement”,i.e. for providing support against the input and output shafts beingdrawn apart; or as a bidirectional bearing arrangement providing supportin both axial directions.

According to an embodiment, said axially rigid support structure is amain thrust bearing support axially supporting said input shaft and saidoutput shaft, such that said axial thrusts at least partly cancel outvia said main thrust bearing support. The main thrust bearing supportmay be any axially rigid structure holding both the input shaft and theoutput shaft in respective thrust bearings, so as to allow therespective axial thrusts to cancel out. By way of example, the mainthrust bearing support may be a centrally located bracket in a gearunit, said bracket holding the input and output shafts such that thedriven gear of the output shaft and the drive gear of the input shaftare located on opposite sides of said bracket. Each of said input andoutput shafts will thereby either simultaneously pull or simultaneouslypush onto opposite sides of said main thrust bearing support, such thatsaid thrusts will at least partly cancel out. Alternatively, the mainthrust bearing support may be a portion of an end wall of a gear unithousing, with respect to which wall the driven gear of the output shaftand the drive gear of the input shaft may be arranged on the same side.In such a configuration, one of said shafts will apply an axiallypulling force onto said wall, while the other shaft will apply anaxially pushing force, such that said thrusts will at least partlycancel out in said wall.

According to an embodiment, said method further comprises

transmitting said rotary motion via at least one layshaft of saidlayshaft arrangement; and

for each layshaft of said layshaft arrangement,

-   -   generating a drive gear axial thrust in a helical drive gear        having a drive gear helix angle;    -   generating a driven gear axial thrust in a helical driven gear        having a driven gear helix angle, said driven gear helix angle        being larger than said drive gear helix angle;    -   directing the drive gear axial thrust in said second direction;        and    -   directing the driven gear axial thrust in said first direction,        such that the axial thrust of the respective drive and driven        gears of each layshaft at least partly cancel out within said        layshaft. By canceling also at least a portion of the axial        thrusts acting on each layshaft, the total resultant axial loads        on a gear unit will be reduced. According to one embodiment the        layshaft arrangement comprises a plurality of layshafts, wherein        the axial thrusts of each of said plurality of layshafts at        least partly cancel out within each layshaft in accordance with        the principles described hereinabove.

According to another aspect of the invention, parts or all of the abovementioned problems are solved, or at least mitigated, by a mobilevehicle gear unit comprising an output shaft, and an input shaftsubstantially parallel with said output shaft, the gear unit beingconfigured for providing a transmission ratio between said input shaftand said output shaft via a layshaft arrangement, the output shaft beingprovided with a driven gear in mesh with a drive gear of said layshaftarrangement, and the input shaft being provided with a drive gear inmesh with a driven gear of said layshaft arrangement, the transmissionratio being non-unity such that one gear of the driven gear of theoutput shaft and the drive gear of the input shaft will be arranged foroperating at a relatively lower torque, and the other gear will bearranged for operating at a relatively higher torque, said relativelyhigher torque being higher than said relatively lower torque; the outputshaft being journalled in an output shaft main thrust bearingarrangement, mounted to a main thrust bearing support, and beingarranged for limiting axial movement of the output shaft in a firstaxial direction;

the input shaft being journalled in an input shaft main thrust bearingarrangement, said input shaft main thrust bearing arrangement beingco-located with said output shaft main thrust bearing arrangement onsaid main thrust bearing support, said input shaft main thrust bearingarrangement being arranged for limiting axial movement of the inputshaft in a second axial direction, said second axial direction beingsubstantially opposite to said first axial direction; said main thrustbearing support rigidly connecting the output shaft main thrust bearingarrangement to the input shaft main thrust bearing arrangement; saiddriven gear of said output shaft being helical of a first hand; saiddrive gear of said input shaft being helical of a second hand, saidsecond hand being the same as the first hand for a positive transmissionratio, i.e., when the input and output shafts rotate in the samedirection, and opposite to said first hand for a negative transmissionratio, i.e., when the input and output shafts rotate in differentdirections; and

said gear arranged for operating at a relatively lower torque having ahelix angle exceeding the helix angle of said gear arranged foroperating at a relatively higher torque.

In such a gear unit, when torque is supplied to the input shaft in adrive direction, axial thrust generated by the driven gear of the outputshaft will be directed in said first axial direction, against saidoutput shaft main thrust bearing arrangement. Axial thrust generated bythe drive gear of the input shaft will be directed in a second axialdirection, which is substantially opposite to said first axialdirection. Hence, input and output shaft axial thrusts will be ofopposite directions and applied to the same location, i.e. where theinput and output shaft main thrust bearing arrangements are joined bythe main thrust bearing support. Hence, such a gear unit may be used forcarrying out the method described hereinbefore, and thereby relates tothe same inventive concept. The difference in helix angles will operateso as to make the magnitudes of said thrusts more equal, so as toincrease the extent to which said axial thrusts at least partly canceleach other out in the main thrust bearing support. The resultant forceon the bearing support will thereby be limited. Thanks to the input andoutput shafts being arranged for applying their respective axial thruststo the same, axially rigid location, axial movement of the shafts isreduced. This allows for tighter gearing backlash, which prolongsservice life of the mobile vehicle gear unit. Furthermore, reduced axialmovement reduces so-called skidding, a phenomenon that will be describedin more detail below. Similar to “substantially opposite directions”,the term “substantially parallel” is to be construed as forming an angleof less than 15°.

According to an embodiment, said main thrust bearing support is fixed toa gear unit housing. Thereby, the main thrust bearing support mayperform the additional function of providing radial support.

According to an embodiment, said output shaft main thrust bearingarrangement is arranged on a first side of said main thrust bearingsupport, and said input shaft main thrust bearing arrangement isarranged on a second side of said main thrust bearing support, saidsecond side being opposite to said first side.

According to another aspect of the invention, parts or all of the abovementioned problems are solved, or at least mitigated, by a mobilevehicle gear unit comprising an input shaft, and an output shaftsubstantially parallel with said input shaft, the gear unit beingconfigured for providing a transmission ratio between said input shaftand said output shaft via a layshaft arrangement, the output shaft beingprovided with a driven gear in mesh with a drive gear of said layshaftarrangement, and the input shaft being provided with a drive gear inmesh with a driven gear of said layshaft arrangement, the transmissionratio being non-unity such that one gear of the driven gear of theoutput shaft and the drive gear of the input shaft will be arranged foroperating at a relatively lower torque, and the other gear will bearranged for operating at a relatively higher torque, said relativelyhigher torque being higher than said relatively lower torque; the inputshaft being journalled to the output shaft in a main thrust bearingarrangement arranged for limiting axial movement of the input shaftrelative to the output shaft in a first axial direction; said drivengear of said output shaft being helical of a first hand; said drive gearof said input shaft being helical of a second hand, said second handbeing the same as the first hand for a positive transmission ratio andopposite to said first hand for a negative transmission ratio; and saidgear arranged for operating at a relatively lower torque having a helixangle exceeding the helix angle of said gear arranged for operating at arelatively higher torque. In such a gear unit, when torque is suppliedto the input shaft in an input direction, axial thrust generated by thedriven gear of the output shaft will be directed in said first axialdirection. Axial thrust generated by the drive gear of the input shaftwill be directed in a second axial direction, which is substantiallyopposite to said first axial direction. Hence, input and output shaftaxial thrusts will be of opposite directions and applied to the samelocation, i.e. where the shafts are joined by the main thrust bearingarrangement. Such a gear unit may be used for carrying out the methoddescribed hereinbefore, and thereby relates to the same inventiveconcept. The difference in helix angles will operate so as to make themagnitudes of said thrusts more equal, so as to increase the extent towhich said axial thrusts at least partly cancel each other out in saidmain thrust bearing arrangement. The resultant axial thrust acting onthe input and output shafts will thereby be limited. Thanks to the inputand output shafts being arranged for applying their respective axialthrusts to the same, axially non-yielding main thrust bearingarrangement, axial and radial movement of the shafts is reduced. Thisallows for tighter gearing backlash, which prolongs service life of thegear unit. Furthermore, reduced axial movement reduces rolling elementskidding in the axial thrust bearings.

According to an embodiment, said output shaft is arranged on a firstside of said main thrust bearing arrangement, and said input shaft isarranged on a second side of said main thrust bearing arrangement, saidsecond side being opposite to said first side.

According to an embodiment of any of the gear units describedhereinbefore, each of said main thrust bearing arrangements is abidirectional thrust bearing arrangement for limiting axial movement ofthe respective input or output shaft in two axial directions. Thereby,axial forces cancel out in the main thrust bearing support or the mainthrust bearing, as the case may be, regardless of the direction ofrotation or input torque of the gear unit.

According to an embodiment of any of the gear units describedhereinbefore, said input shaft is connected to a power source, such asan electric motor or an internal combustion engine, for driving theinput shaft in an input direction of rotation, and the hand of saiddriven gear of said output shaft is oriented for applying output shaftaxial thrust in said first axial direction when the power sourcetransmits torque to the input shaft in said input direction. Implicitly,the axial thrust of the output shaft will thereby be applied in saidsecond axial direction substantially opposite to said first axialdirection. By having the mobile vehicle gear unit connected to a powersource, for receiving rotary power primarily in said input directiontherefrom, it is possible to design the gear unit so as to withstand alarger torque in the predetermined input direction than in a directionof rotation opposite to said input direction. Thereby, weight may besaved and the construction may be simpler, since thrust bearing(s) maybe unidirectional, and/or housing walls may be made thinner.

According to an embodiment of any of the gear units describedhereinbefore, each of said input and output shafts is axially preloadedin a preloading arrangement. Each of said input and output shafts may bepreloaded in a preload arrangement either between a pair of auxiliarypreload bearings, or between a main thrust bearing arrangement and anauxiliary preload bearing. In the latter case, the main thrust bearingarrangement will have the double function of cancelling out dynamicaxial loads, and acting as one of the preload bearings of a preloadingarrangement. The auxiliary preload bearings may be located e.g. at therespective end walls of a gear unit housing. By directing the dynamicaxial thrust to a single, axially rigid location, where the thrusts atleast partly cancel out when the gear unit is driven in a forward,high-load drive direction, the magnitude of the preload force can beselected more freely and with a higher accuracy. In the case of taperedroller bearings, an accurate axial preloading reduces skidding andbrings a greater portion of a bearing's rolling elements in contact withbearing inner and outer races during a greater portion of each turn ofthe respective shaft, thereby sharing the axial load more accuratelybetween the rolling elements. For this reason, a correctly preloadedthrust bearing generally has a longer life expectancy than anon-preloaded thrust bearing. However, a too highly preloaded thrustbearing generally has a shorter life expectancy than a non-preloadedbearing. By cancelling out a significant portion of the dynamic axialforces, the preload of the shafts can be kept on a lower, more constant,and more accurately selectable level. Furthermore, any housing orsupport structure carrying the preload thrust bearings may bedimensioned rigid enough for the static preload, without becomingexcessively heavy as it would have been if required to take up heavydynamic axial thrusts originating from operation of helical gears aswell. It also becomes possible to arrange the preload thrust bearings ina flexible support so as to provide a selected, constant axialpreloading force. Still further, axial preloading also to some extentreduces the axial and radial movement of the respective shafts, therebyprolonging service life expectancy.

According to an embodiment of any of the gear units describedhereinbefore, said input and output shafts are substantially concentric.Thereby, a minimum of transversal torque will act on the main thrustbearing main thrust bearing support, and minimum of bending force willact on the output shaft. Throughout this disclosure, “substantiallyconcentric” is to be construed as a central axis of at least one of theinput shaft and the output shaft extending through a surface defined bythe outer boundaries of a thrust bearing supporting the other shaft.

According to an embodiment of any of the gear units describedhereinbefore, said driven gear of said output shaft has an output shaftdriven gear pitch diameter D_(out) and an output shaft driven gear helixangle ψ_(out);

said drive gear of said input shaft has an input shaft drive gear pitchdiameter D_(in) and an input shaft drive gear helix angle ψ_(in); and

said driven gear of said output shaft and said drive gear of said inputshaft satisfy a condition corresponding to

${0.2 < {{I_{tot}\frac{D_{in}\tan \; \Psi_{out}}{D_{out}\tan \; \Psi_{in}}}} < 5},$

wherein I_(tot) is the transmission ratio of said gear unit. Thereby,the resultant axial force is even further reduced. Preferably,

${0.5 < {{I_{tot}\frac{D_{in}\tan \; \Psi_{out}}{D_{out}\tan \; \Psi_{in}}}} < 2},$

such that input and output axial thrusts to an even greater extentcancel out in the main thrust bearing or the main thrust bearingsupport, as the case may be.

According to an embodiment of any of the gear units describedhereinbefore, said layshaft arrangement comprises at least one layshaft,and optionally a plurality of substantially parallel layshafts connectedin series, each layshaft being provided with a helical drive gear and ahelical driven gear, the driven gear of each layshaft being of the samehand as the drive gear of the same layshaft. Thereby, axial thrusts ofeach layshaft at least partly cancel out as a compressive or tensileaxial force within each respective layshaft. Preferably, for eachlayshaft i of said at least one layshaft, or of said optional pluralityof layshafts, the respective drive gear has a drive gear pitch diameterD_(drive, i) and a drive gear helix angle ψ_(drive, i); the respectivedriven gear has a driven gear pitch diameter D_(driven, i) and a drivengear helix angle ψ_(driven, i), the driven gear helix angleψ_(driven, i) being different from the drive gear helix angleψ_(drive, i); and

0.2<|(D _(drive, i)*tan ψ_(driven, i))/(D _(driven, i)*tanψ_(drive, i))|<5.

Under this particular condition, the axial thrust components acting oneach layshaft cancel each other out to an even greater extent. Even morepreferably,

0.5<|(D _(drive, i)*tan ψ_(driven, i))/(D _(driven, i)*tanψ_(drive, i))|<2,

such that most of the axial thrust components acting on each layshaftcancel out. It will be appreciated that ψ_(drive, i) does not need to beidentical to ψ_(driven, i−1); such may be the case e.g. if therespective shafts are not exactly parallel.

According to an embodiment of any of the gear units describedhereinbefore, said gear unit is a step-down gear. And according to anembodiment of any of the gear units described hereinbefore, said gearunit has a transmission ratio, between said input shaft and said outputshaft, of 30:1 or less. Preferably the transmission ratio is in therange of 20:1 to 0.5:1, and more preferably in the range of 15:1 to0.5:1. The gear unit designs disclosed herein are particularly wellsuited for the large output torques of a step-down gear and/or ahigh-transmission ratio gear unit.

According to an embodiment of any of the gear units describedhereinbefore, said gear unit has a fixed transmission ratio. Such adesign is relatively compact and reliable, making it well suited forelectric vehicles having electric motors that do not require anypossibility to vary the gear ratio between the electric motor and thedrive wheels.

According to another embodiment the gear unit has a variabletransmission ratio. Such a gear unit is particularly well suited forelectric drive vehicles with high top speed, and vehicles with internalcombustion engines.

According to an embodiment, the driven gear of the output shaft isaxially fixed to the output shaft, and the drive gear of the input shaftis axially fixed to the input shaft.

According to an embodiment, for each layshaft of said layshaftarrangement, the driven gear of the layshaft is axially fixed relativeto the drive gear of the layshaft.

According to an embodiment, for each shaft of said gear unit, theangular shaft play e relative to another shaft of said gear unitsatisfies the condition

θ_(S)<tan⁻¹(0.11/n _(G))

wherein n_(G) represents the number of teeth of a gear of said shaft,said gear being in engagement with a gear of said another shaft.

According to one embodiment the output shaft is tapering inwardly in adirection toward a main thrust bearing arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of thepresent invention, will be better understood through the followingillustrative and non-limiting detailed description of preferredembodiments of the present invention, with reference to the appendeddrawings, where the same reference numerals will be used for similarelements, wherein:

FIG. 1 is a schematic side view of an electric drive vehicle;

FIG. 2 is a schematic side view of the drive unit of the electric drivevehicle of FIG. 1;

FIG. 3 is a schematic view in perspective of gearing of a mobile vehiclegear unit;

FIG. 4 is a schematic top view in section of a mobile vehicle gear unitcomprising the gearing of FIG. 3;

FIG. 5 a is schematic cross-section side view, taken along the line V-V,of the gear unit of FIG. 4;

FIG. 5 b is a magnified view of the area of FIG. 5 a defined by a dashedrectangle;

FIG. 6 a is schematic cross-section side view of a second embodiment ofa gear unit;

FIG. 6 b is a magnified view of the area of FIG. 6 a defined by a dashedrectangle;

FIG. 7 a is schematic cross-section side view of a third embodiment of agear unit;

FIG. 7 b is a magnified view of the area of FIG. 7 a defined by a dashedrectangle;

FIG. 8 a is schematic cross-section top view of a fourth embodiment of agear unit; and

FIG. 8 b is a schematic cross-section side view, taken along the lineB-B, of the gear unit of FIG. 8 a.

FIG. 9 a is schematic side view of a fifth embodiment of a gear unit;

FIG. 9 b is a schematic cross-section side view of the gear unit of FIG.9 a.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Mobile vehicle gear units of prior art have a few weaknesses. By way ofexample, the shafts of a mobile vehicle gear unit are subjected to axialthrust, which is taken up by thrust bearings in the gear unit housing.For example, an electric motor transmits very high levels of torque tothe gear unit already at low rpm; hence, the radial and axial load onthe input shaft bearings may be very high. The high load may cause thegear unit housing to flex or yield, leading to increased axial bearingplay and requiring substantial gearing backlash. Such axial play maylimit the lifetime of the mobile vehicle gear unit; hence, the gear unithousing needs to be designed to take up substantial forces, making itheavy.

The direction of axial thrust varies with the load condition on themobile vehicle gear unit; therefore, axially unconstrained shafts maytranslate in the axial direction depending on the rpm and torquetransmitted from the electric motor. Within a bearing, a loss of contactbetween rolling elements and a race surface may result in intermittent,skidding contact between race surface and rolling elements. Thisphenomenon is called skidding, and may increase bearing wear. Stillfurther, alternating torsion transients, occurring due to substantialbacklash, i.e. clearance between the teeth of meshing gears, alsocontribute to shorten the lifetime of a mobile vehicle gear unit.

FIG. 1 schematically illustrates a vehicle 10 comprising a car body 12and a drive unit 14. The drive unit 14 comprises a motor 16. The motor16 may, for example, be an electric motor, a displacement engine, forexample an internal combustion engine, such as petrol or diesel engine,or a dynamic engine, such as jet engine. The drive unit 14 furthercomprises an energy storage system 18, which may be a battery pack ifthe motor 16 is an electric motor, and a fuel tank if the motor 16 is aninternal combustion engine, a drive shaft 20 for transferring rotationto drive wheels 22 of the vehicle 10, and a mobile vehicle gear unit 24connecting the motor 16 to the drive shaft 20. A typical rotation speedof the motor 16 may, by way of example, be between 1000 and 20 000 rpm.

FIG. 2 schematically illustrates the mobile vehicle gear unit 24. Themotor 16 is connected to the drive wheels 22 (FIG. 1) via the gear unit24, which steps down the high rotation speed of the motor 16 to a lowerspeed of the drive shaft 20, and of the drive wheels 22. A gear unitoutput shaft 26 interconnects the gear unit 24 with the drive shaft 20and the drive wheels 22, and provides a low-speed, high-torque rotarymotion, in a driving direction 19, to the drive wheels 22, which convertthe rotary motion to a forward movement of the vehicle 10. A gear unitinput shaft 28 interconnects the motor 16 with the gear unit 24, andprovides a high-speed, low-torque rotary motion to the gear unit 24.

Typical drive wheels 22 may, by way of example, be designed foroperating at a rotation speed of between 100 and 5000 rpm (revolutionsper minute). The total transmission ratio I_(tot) of the gear unit 24can be defined as the signed rotation speed of the input shaft 28divided by the signed rotation speed of the output shaft 26, when bothshafts 28, 26 are viewed in a direction from the input shaft 28 to theoutput shaft 26. When the input shaft 28 rotates in one direction, andthe output shaft 26 rotates in the same direction, i.e., the sign of theinput shaft 28 rotation is the same as the sign of the output shaft 26rotation, then the total transmission ratio I_(tot) will be positive(dividing numbers of the same sign), and when the input shaft 28 rotatesin one direction, and the output shaft 26 rotates in another direction,i.e., the sign of the input shaft 28 rotation is different from the signof the output shaft 26 rotation, then the total transmission ratioI_(tot) will be negative (dividing numbers of different sign). Hence,the total transmission ratio I_(tot) of the gear unit 24 is preferably+/−30:1 or less.

FIG. 3 illustrates the gearing of the gear unit 24 in greater detail.The output shaft 26 is connected to the input shaft 28 via a layshaftarrangement 29 comprising a first layshaft 30 and a second layshaft 32,which are connected in series. The output shaft 26 is provided with adriven gear 34, which is in mesh with a drive gear 36 on the firstlayshaft 30. The driven gear 34 of the output shaft 26 has a largerpitch diameter D_(out) than the corresponding pitch diameterD_(drive, 1) of the drive gear 36 of the first layshaft 30, such thatthe engagement of the driven gear 34 of the output shaft 26 with thedrive gear 36 of the first layshaft 30 provides a first step-down, oftransmission ratio of the rotary speed from the first layshaft 30 to theoutput shaft 26.

The first layshaft 30 is provided with a driven gear 38, which is inmesh with a drive gear 40 on the second layshaft 32. The driven gear 38of the first layshaft 30 has a larger pitch diameter D_(driven, 1) thanthe corresponding pitch diameter D_(drive, 2) of the drive gear 40 ofthe second layshaft 32, such that the engagement of the driven gear 38of the first layshaft 30 with the drive gear 40 of the second layshaft32 provides a second step-down, of transmission ratio I₂, of the rotaryspeed from the second layshaft 32 to the first layshaft 30.

The second layshaft 32 is provided with a driven gear 42, which is inmesh with a drive gear 44 on the input shaft 28. The driven gear 42 ofthe second layshaft 32 has a larger pitch diameter D_(driven, 2) thanthe corresponding pitch diameter D_(in) of the drive gear 44 of theinput shaft 28, such that the engagement of the driven gear 42 of thesecond layshaft 32 with the drive gear 44 of the input shaft 28 providesa third step-down, of transmission ratio I₃, of the rotary speed fromthe input shaft 28 to the second layshaft 32. Hence, the gear unit 24comprises three gear steps of transmission ratios I₂, I₃, which providea total transmission ratio I_(tot)=I₁*I₂*I₃ from the input shaft 28 tothe output shaft 26.

FIG. 4 illustrates the mobile vehicle gear unit 24 arranged in a gearunit housing 45.

The driven gear 34 of the output shaft 26 is a helical gear of a firsthand, said first hand in this example being right hand, right hand beingdefined as the teeth twisting clockwise as they recede from an observerlooking along the axis A_(out) of the driven gear 34 of the output shaft26. The driven gear 34 of the output shaft 26 further has a helix angleψ_(out), defined as the unsigned value of the angle formed between atangent to the gear's helix at the pitch circle, and the direction ofthe central axis A_(out) of the driven gear 34.

The drive gear 44 of the input shaft 28 is a helical gear of a secondhand, said second hand in this example being left hand, defined as theteeth twisting counter-clockwise as they recede from an observer lookingalong the axis A_(in) of the drive gear 44 of the input shaft 28. Thedrive gear 44 of the input shaft 28 further has a helix angle ψ_(in),defined as the unsigned value of the angle formed between a tangent tothe gear's helix at the pitch circle, and the direction of the centralaxis A_(in) of the drive gear 44. The same definition of helix angleapplies, mutatis mutandis, to the other helical gears of the gear unit24.

When the output shaft 26 is rotated in the driving direction 19 (FIG.3), said driving direction being clockwise, as seen in the direction ofthe central axis A_(out) of the output shaft 26 towards the input shaft28, the helical driven gear 34 of the output shaft 26, which is axiallyfixed to the output shaft 26, will generate an axial thrust F1 acting onthe output shaft 26. Due to the hand of the driven gear 34 of the outputshaft 26, the axial thrust F1 will be directed towards the input shaft28.

The drive gear 44 of the input shaft 28, which, via said first andsecond layshafts 30, 32, will rotate in an input direction 47 (FIG. 3)opposite to the driving direction 19, will generate an axial thrust F2.The drive gear 44 of the input shaft 28 is axially fixed to the inputshaft 28, such that the axial force F2 will act on the input shaft 28.Due to the hand of the drive gear 44 of the input shaft 28, the axialthrust F2 will be directed towards the output shaft 26. The directionsof the axial thrusts F1, F2 are illustrated by arrows.

The output shaft 26 and the input shaft 28 are concentric, and meet at amain thrust bearing support 50. At the main thrust bearing support 50,the output and input shafts 26, 28 are journalled in a manner which willbe described in more detail further below. The main thrust bearingsupport 50 is fixed to the gear unit housing 45 in a non-rotatingmanner, and in this particular example forms a bracket for holding theoutput shaft 26 on a first, output side 52 thereof, and the input shaft28 on a second, input side 54 thereof. The output side 52 of the mainthrust bearing support 50 is opposite to the input side 54.

The main thrust bearing support 50 forms an axially rigid supportstructure, at which the output and input shafts 26, 28 are journalled soas to transfer the axial thrusts F1, F2 thereto. Thanks to therespective hands of the driven gear 34 of the output shaft 26, and ofthe drive gear 44 of the input shaft 28, being oriented such that therespective axial thrusts F1, F2 are directed in opposite directions, theaxial thrusts F1, F2 will thereby at least partly cancel out in the mainthrust bearing support 50. This reduces movement of the output and inputshafts 26, 28 and their associated gears 34, 44, which in turn allowsfor tighter gearing backlash. And thanks to the output and input shafts26, 28 being concentric, the axial thrusts F1, F2 will not result in anysubstantial transversal torque or bending force onto the main thrustbearing support 50, or onto the shafts 26, 28 themselves.

When torque is supplied to the input shaft 28 in the input direction 47(FIG. 3), the axial thrusts F1, F2 cancel out as a compressive forcewhere the output and input shafts 26, 28 meet in the main thrust bearingsupport 50. During normal operation of a vehicle 10 (FIG. 1), theaverage and instantaneous torque supplied to the mobile vehicle gearunit 24 is typically larger in the input direction 47, i.e. forwardmovement, than in the opposite direction. The opposite direction may,for example, be used when the vehicle 10 (FIG. 1) is to be reversed.Reversing an electric drive vehicle may be made by simply shifting therotational direction of the electric motor, to a direction which isopposite to the input direction 47. Reversing the vehicle is usuallymade for very short periods of time, and at rather low load. Hence, themobile vehicle gear unit 24 does not necessarily need to be arranged fortaking up heavy axial thrusts in the opposite directions, i.e. thrustsacting so as to urge the output shaft 26 away from the input shaft 28.

According to the definition of transmission ratio I_(tot) hereinbefore,opposite directions of rotation of the output shaft 26 and the inputshaft 28 will yield a negative total transmission ratio I_(tot). As ageneral rule, in order to generate an output shaft axial thrust F2 in adirection opposite to the input shaft axial thrust F1, the second hand,said second hand being the hand of the input shaft 28, should be thesame as the first hand, said first hand being the hand of the outputshaft 26, for an even number of layshafts connected in series betweenthe output shaft 26 and the input shaft 28. For an odd number oflayshafts connected in series between the output shaft 26 and the inputshaft 28, the second hand should be the same as the first hand.Furthermore, for a clockwise driving direction 19 of the output shaft26, as seen in the axial direction towards the input shaft 28, aright-hand driven gear 34 of the output shaft 26 should be selected forobtaining axial thrusts F1, F2 that meet at the main thrust bearingsupport 50. For a counter-clockwise driving direction 19 of the outputshaft 26, a left-hand driven gear 34 of the output shaft 26 should beselected for obtaining axial thrusts F1, F2 that meet at the main thrustbearing support 50.

Preferably, the driven gear 34 of the output shaft 26 and the drive gear44 of the input shaft 28 satisfy the condition:

$\begin{matrix}{0.2 < {{I_{tot}\frac{D_{in}\tan \; \Psi_{out}}{D_{out}\tan \; \Psi_{in}}}} < 5} & (1)\end{matrix}$

It has been found that if this condition is satisfied, a significantportion of the axial thrusts F1, F2 of the output and input shafts 26,28 cancel out.

More preferably,

$\begin{matrix}{{0.5 < {{I_{tot}\frac{D_{in}\tan \; \Psi_{out}}{D_{out}\tan \; \Psi_{in}}}} < 2},} & (2)\end{matrix}$

and ideally,

$\begin{matrix}{{{{I_{tot}\frac{D_{in}\tan \; \Psi_{out}}{D_{out}\tan \; \Psi_{in}}}} \approx 1},} & (3)\end{matrix}$

such that there is an almost complete axial thrust balance between F1and F2. Thereby, the axial load exerted by the input and output shafts28, 26 onto the axial end walls of the gear unit housing 45 can beessentially eliminated. As is apparent to those skilled in the art, theexpressions (1)-(3) above may also be adjusted so as to compensate forthe friction of the mobile vehicle gear unit 24.

As a specific example fulfilling all the above conditions, for themobile vehicle gear unit 24 having a total transmission ratio I_(tot) of2,5:1, the driven gear of the output shaft 26 may have a pitch diameterD_(out) of 110 mm (millimeters) and a helix angle ψ_(out) of 12°,whereas the drive gear 44 of the input shaft 28 may have a pitchdiameter D_(in) of 60 mm and a helix angle ψ_(in) of 16°.

In order to achieve a minimum total axial load onto the gear unithousing 45, also the layshafts 30, 32 of the layshaft arrangement 29(FIG. 3) may be axially balanced. This may be achieved by each of thelayshafts 30, 32 being equipped with a respective driven gear and arespective drive gear of the same hand, preferably with a larger helixangle on its respective larger pitch diameter gear than on itsrespective smaller pitch diameter gear. In the exemplary mobile vehiclegear unit 24 of FIG. 4, the drive gear 36 of the first layshaft 30 has adrive gear helix angle ψ_(drive, 1) and the driven gear 38 has a drivengear helix angle ψ_(driven, 1) of the same hand as the drive gear 36.Thereby, when the output shaft 26 is rotated in said driving direction19, at least a portion of the axial thrusts generated by the helicalgears 36, 38 of the first layshaft 30 will cancel out as a tensile forcein the first layshaft 30. The pitch diameter D_(drive, 1) of the drivegear 36 is larger than the pitch diameter D_(driven, 1) of the drivengear 38; hence, the driven gear helix angle ψ_(driven1) preferablyexceeds the drive gear helix angle ψ_(drive, 1), such that axial balanceis still further improved.

Preferably, for each layshaft i of a layshaft arrangement,

0.2<|(D _(drive,i)*tan ψ_(driven,i))/(D _(driven,i)*tan ψ_(drive,i))|<5,  (4)

where, in the specific embodiment of FIG. 4, i=1 represents therespective properties D_(driven,1), ψ_(driven,1), D_(drive,1), andψ_(drive,1) of the first layshaft 30, and i=2 represents the respectiveproperties of the second layshaft 32. More preferably,

0.5<|(D _(drive, i)*tan ψ_(driven, i))/(D _(driven, i)*tanψ_(drive, i))|<2,   (5)

and ideally,

|(D_(drive, i)*tan ψ_(driven, i))/(D_(driven, i)*tan ψ_(drive, i))|≈1.  (6)

Under those conditions, the layshafts 30, 32 may be completely balancedin the axial direction, and their journaling may be thrust bearing free,i.e. they may be journalled for substantive support only in the radialdirection.

As a specific example fulfilling all the above conditions, for themobile vehicle gear unit 24 of FIG. 3, the drive gear 36 of the firstlayshaft 30 may have a pitch diameter D_(drive,1) of 80 mm and a helixangle ψ_(drive,1) of 12°; the driven gear 38 of the first layshaft 30may have a pitch diameter D_(driven,1) of 100 mm and a helix angleψ_(driven,1) of 15°; the drive gear 40 of the second layshaft 32 mayhave a pitch diameter D_(drive,2) of 75 mm and a helix angle ψ_(drive.2)of 15°; and the driven gear 42 of the second layshaft 32 may have apitch diameter D_(driven,2) of 80 mm and a helix angle ψ_(driven.2) of16°.

FIG. 5 a, showing the section V-V along the central axis A_(out) of FIG.4, illustrates an exemplary journaling of the output and input shafts26, 28, and the magnified view of FIG. 5 b illustrates the details ofhow the shafts 26, 28 are journalled to the main thrust bearing support50. The output shaft 26 is journalled to the main thrust bearing support50 in an output shaft main thrust bearing arrangement 56, whichcomprises a first thrust bearing, embodied as a first tapered rollerbearing 58, and a second thrust bearing, embodied as a second taperedroller bearing 60. The two tapered roller bearings 58, 60 taper inopposite directions, such that they together form a bidirectional thrustbearing arrangement, i.e. the output shaft thrust bearing arrangement 56is adapted for supporting substantial axial loads in both axialdirections.

Similarly, the input shaft 28 is journalled to the main thrust bearingsupport 50 in an input shaft main thrust bearing arrangement 62, whichalso comprises two tapered roller bearings 64, 66 tapering in oppositedirections, thereby forming a bidirectional thrust bearing arrangement.The output and input main thrust bearing arrangements 56, 62 areco-located on the main thrust bearing support 50, which interconnectssaid main thrust bearing arrangements 56, 62 in an axially rigid manner.

Thanks to the output and input shaft main thrust bearing arrangements56, 62 being bidirectional, when torque is supplied to the input shaft28 in a direction opposite to said input direction 47, the axial thrustsF1, F2 cancel out as a tensile force in the main thrust bearing support50. Thereby, axial loads onto axial end walls 78, 80 of the gear unithousing 45 are reduced regardless of the direction of operation or theload direction of the input shaft 28.

An auxiliary bearing 68 supports the output shaft 26 in the radialdirection. Since the output shaft main thrust bearing arrangement 56 isbidirectional and provides all axial support that is needed, theauxiliary bearing 68 does not need to be arranged for providing anyaxial support. Hence, the auxiliary bearing 68 may be a simple, radiallysupporting bearing of e.g. the cylindrical, non-tapered roller bearingtype.

Alternatively, also the auxiliary bearing 68 may be an axial thrustbearing, e.g. of the tapered roller bearing type, which supports theoutput shaft 26 in an axial direction. Thereby, the auxiliary bearing 68may be used as a preload bearing for axially preloading the output shaft26 between the auxiliary bearing 68 and the output shaft main thrustbearing arrangement 56. In such a configuration, the auxiliary bearing68 and the output shaft main thrust bearing arrangement 56 together forma preloading arrangement, which may permanently keep the output shaft 26under tensile or compressive load. Thereby, the lifetime of the outputshaft main thrust bearing arrangement 56 may be extended. Furthermore, aminimum of backlash can be designed into the mating of the teeth of thegears of the mobile vehicle gear unit 24, such that the operationallifetime of the entire gear unit 24 is increased.

Also the input shaft 28 may be preloaded in a preloading arrangementformed by an auxiliary axial thrust bearing 70 and the input shaft mainthrust bearing arrangement 62. It is also possible to accurately preloadthe layshafts 30, 32, which may be axially balanced in line with whathas been described hereinbefore with reference to FIG. 4, in similarpreloading arrangements of thrust bearings.

FIGS. 6 a-b illustrate an alternative configuration of the bearings ofthe mobile vehicle gear unit 24, according to which configuration theoutput shaft 26 and the input shaft 28 are arranged for transferringaxial thrusts F1, F2 more directly to each other. In the particularexample illustrated in FIG. 6 a-b, the input shaft 28 is journalled tothe output shaft 26 via a main thrust bearing arrangement 156, whichallows the output and input shafts 26, 28 to rotate independently ofeach other. The main thrust bearing arrangement 156 comprises a firsttapered roller bearing 158 and a second tapered roller bearing 160,which are arranged in an axial recess 174 of the output shaft 26. Thetwo tapered roller bearings 158, 160 taper in opposite directions, suchthat they together form a bidirectional thrust bearing arrangement.Thereby, the axial thrusts F1, F2 of the output and input shafts 26, 28meet and at least partly cancel each other out at the main thrustbearing arrangement 156 irrespective of the input direction of the inputshaft 28. The main thrust bearing arrangement 156 thereby forms anaxially rigid support structure for non-yieldingly receiving axial loadsfrom both the output shaft 26 and the input shaft 28.

The output shaft 26 is journalled in an auxiliary bearing 176, which ismounted on a bearing support 150. The auxiliary bearing 176 does notneed to be a thrust bearing, since axial thrust will mainly be borne bythe main thrust bearing arrangement 156. Similar to what has beendescribed above with reference to FIG. 5 a-b, however, the auxiliarybearing 176 may, as an alternative, be a thrust bearing that can be usedfor preloading the output shaft 26 against e.g. a second auxiliarybearing 68.

Neither the bearing support 150 nor the auxiliary bearing 176 mountedthereto are necessary for balancing the dynamic axial thrusts that occurwhen operating the gear unit 24; hence, they can be dispensed with, andradial support may be provided by other means available to the personskilled in the art.

As an alternative to incorporating into the mobile vehicle gear unit 24a main thrust bearing arrangement 156 that is bidirectional, for a gearunit that is intended for an application in which it is mainly exposedto high torques in a single, predetermined input direction 47 (FIG. 3),it would be sufficient to use a unidirectional main thrust bearingarrangement as has been described in the foregoing.

FIGS. 7 a-b illustrate yet an alternative configuration of the bearingsof the mobile vehicle gear unit 24, according to which configuration theoutput shaft 26 is hollow, and the main thrust bearing arrangement 156is located deep into the recess 174 of the output shaft 26. Even though,in FIG. 7 a, the axial thrust F1 is illustrated on the input shaft 28,it will be appreciated that the thrust F1 acts on the hollow outputshaft 26 surrounding the input shaft 28.

In the configuration of FIGS. 7 a-b, the output and input shafts 26, 28are interconnected via a main thrust bearing arrangement 156, via whichthe axial thrusts F1, F2 of the shafts 26, 28 cancel out. However, as analternative (not shown), the output shaft 26 may be journalled to theaxial end wall 178 of the gear unit housing 45 in an output shaft mainthrust bearing arrangement, which is configured to transfer axial thrustfrom the output shaft 26 to the axial end wall 178. Similarly, the inputshaft 28 may be journalled to the same axial end wall 178 of the gearunit housing 45 in an input shaft main thrust bearing arrangement, whichis configured to transfer axial thrust from the input shaft 28 to theaxial end wall 178. In such a configuration, the axial end wall 178 ofthe gear unit housing 45 would form a main thrust bearing supportsimilar to what has been described hereinbefore with reference to FIG. 5a-b. Such a configuration would however differ from the arrangement ofFIG. 5 a-b in that the driven gear 34 of the output shaft 26 and thedrive gear 44 of the input shaft 28 would be located on the same side ofthe main thrust bearing support, such that one of the output and inputshaft main thrust bearing arrangements would support an axially pushingforce, while the other would support an axially pulling force, when thegear unit 24 is operated.

FIGS. 8 a-b illustrate an alternative configuration of the layshafts,according to which configuration the mobile vehicle gear unit 24comprises a first layshaft arrangement 29 and a second layshaftarrangement 29′. Each of the layshaft arrangements 29, 29′ comprises arespective first layshaft 30, 30′ provided with a drive gear 36, 36′ anda driven gear 38, 38′, and a respective second layshaft 32, 32′, saidsecond layshaft 32, 32′ also being provided with a drive gear 40, 40′and a driven gear 42, 42′. The two layshaft arrangements 29, 29′ arearranged in parallel and configured to provide an identical transmissionratio I_(tot) between the input shaft 28 and the output shaft 26. Theoutput and input shafts 26, 28, as well as the layshafts 30, 30′, 32,32′ of the gear unit 24 will be axially balanced under the sameconditions, defined by the appended claims, as any other of theembodiments disclosed hereinbefore with reference to FIGS. 1-7 b. In theparticular example illustrated in FIG. 8 b, the axial loads of theoutput and input shafts 26, 28 at least partly cancel out in a mainthrust bearing arrangement 156, in a manner similar to what has beendescribed in the foregoing with reference to FIGS. 6 a-b.

Two parallel layshaft arrangements, which are in mesh with the samedriven and drive gears 34, 44, respectively, of the output and inputshafts 26, 28, are illustrated in FIGS. 8 a-b. However, the gear unitmay also be provided with any other number of parallel layshaftarrangements, and the layshafts may be connected between different setsof driven gears of the output shaft 26, and drive gears of the inputshaft 28; the general principles of axial thrust balancing disclosedherein will nevertheless apply.

FIGS. 9 a-b illustrate a mobile vehicle gear unit 24 according to afifth alternative embodiment. The mobile vehicle gear unit 24 of FIGS. 9a-b has a variable total transmission ratio I_(tot). The mobile vehiclegear unit 24 has a first gear, representing a first total transmissionratio I_(tot1) and a second gear, representing a second totaltransmission ratio I_(tot2). The mobile vehicle gear unit 24 of thisfifth embodiment of FIGS. 9 a-b is suitable for electric drive vehicles.An electric drive vehicle is provided with an electric motor 16 (FIG.1). An electric motor 16 has a high torque already at low rpm, e.g. 1000rpm. However, in order to obtain a high top speed and/or a high energyefficiency of the electric drive vehicle the rpm span of the electricmotor 16 of the electric vehicle 10 may not be sufficient. The firstgear of the mobile vehicle gear unit 24 of FIGS. 9 a-b may be used atvehicle speeds of, for example, 0 to 120 kmh, and the second gear of themobile vehicle gear unit 24 of FIGS. 9 a-b may be used at vehicle speedsof, for example, 80 to 200 kmh. Shifting between the first gear and thesecond gear could be made manually, using a gear-change lever, orautomatically, employing, for example, the principles of the per seknown robotized manual gearbox with electronic clutch.

It will be appreciated that alternative mobile vehicle gear unitsmanufactured in accordance with the principles described hereinafterwith reference to FIGS. 9 a-b may be provided with three, four, five,six, seven, or even more gears, providing a further variability in thetotal transmission ratio I_(tot). Furthermore, mobile vehicle gear unitsmanufactured in accordance with the principles described hereinafterwith reference to FIGS. 9 a-b, and provided with anything from two toten gears, or even more, may also be used for other types of motors 16,for example internal combustion engines using, for example, petrol,diesel, biogas, or ethanol as fuel.

Returning to FIGS. 9 a-b the electric drive vehicle mobile vehicle gearunit 24 comprises an output shaft 26 connected to an input shaft 28 viaa layshaft arrangement 29. The layshaft arrangement 29 comprises, inthis embodiment, a single layshaft 30. It will be appreciated that thegear unit 24 may, in alternative embodiments, be provided with two ormore layshafts arranged in series, in accordance with the principles ofFIGS. 3-4, and/or with two or more parallel layshafts, in accordancewith the principles of FIG. 8 a.

The output shaft 26 is provided with a first driven gear 34, which is inmesh with a first drive gear 36 on the layshaft 30, and a second drivengear 35 which is in mesh with a second drive gear 37 on the layshaft 30.In neutral position, as illustrated in FIGS. 9 a-b, the first and seconddriven gears 34, 35 are free-wheeling on the output shaft 26. Agear-change lever 84 may be connected to an engagement ring 86 arrangedon the output shaft 26, between the first and second driven gears 34,35. The engagement ring 86 is axially movable along the output shaft 26,in the axial direction thereof, as illustrated by arrows in FIG. 9 a.The engagement ring 86 is radially locked to the output shaft 26 androtates together with the output shaft 26. By moving the engagement ring86 towards the first driven gear 34, i.e., by moving the engagement ringto the right in FIG. 9 a, the engagement ring 86 may engage the firstdriven gear 34 and radially lock the first driven gear 34, such that thefirst driven gear 34 starts to rotate the output shaft 26. Such radiallocking of the first driven gear 34 to the output shaft 26 is the sameas putting in the first gear of the gear unit 24. By moving theengagement ring 86 towards the second driven gear 35, i.e., by movingthe engagement ring to the left in FIG. 9 a, the engagement ring 86 mayengage the second driven gear 35 and radially lock the second drivengear 35, such that the second driven gear 35 starts to rotate the outputshaft 26. Such radial locking of the second driven gear 35 to the outputshaft 26 is the same as putting in the second gear of the gear unit 24.The engagement of the engagement ring 86 to one of the first and seconddriven gears 34, 35 may be made using the per se known principles ofsynchromesh or dogg engagement.

The first driven gear 34 of the output shaft 26 has a larger pitchdiameter D_(out1) than the corresponding pitch diameter D_(drive, G1) ofthe first drive gear 36 of the layshaft 30, such that the engagement ofthe first driven gear 34 of the output shaft 26 with the first drivegear 36 of the layshaft 30 provides a first gear step-down, oftransmission ratio of the rotary speed from the layshaft 30 to theoutput shaft 26.

The second driven gear 35 of the output shaft 26 has substantially thesame pitch diameter D_(out2) as the corresponding pitch diameterD_(drive, G2) of the second drive gear 37 of the layshaft 30, such thatthe engagement of the second driven gear 35 of the output shaft 26 withthe second drive gear 37 of the layshaft 30 provides a neutral gear, oftransmission ratio I_(G2)=1, of the rotary speed from the layshaft 30 tothe output shaft 26.

The layshaft 30 is provided with a driven gear 38, which is in mesh witha drive gear 44 on the input shaft 28. All gears 36, 37, 38 of thelayshaft 30 are axially locked to the layshaft 30, and transfer axialthrust to the layshaft 30. The driven gear 38 of the layshaft 30 has alarger pitch diameter D_(driven) than the corresponding pitch diameterD_(in) of the drive gear 44 of the input shaft 28, such that theengagement of the driven gear 38 of the layshaft 30 with the drive gear44 of the input shaft 28 provides a step-down, of transmission ratio I₃,of the rotary speed from the input shaft 28 to the layshaft 30.

Hence, in the first gear the mobile vehicle gear unit 24 comprises twogear steps of transmission ratios I_(G1), and I₃, which provide a totalstep-down transmission ratio I_(tot)=I_(G1)*I₃ from the input shaft 28to the output shaft 26, when the gear unit 24 is set in the first gear.In the second gear the mobile vehicle gear unit 24 comprises a singlegear step, since I_(G2)=1, of transmission ratio I₃, which provides atotal step-down transmission ratio I_(tot)=I₃ from the input shaft 28 tothe output shaft 26, when the gear unit 24 is set in the second gear.

The driven gears 34, 35 of the output shaft 26 are each helical gears ofa first hand, said first hand in this example being right hand, righthand being defined as the teeth twisting clockwise as they recede froman observer looking along the axis A_(out) of the driven gears 34, 35 ofthe output shaft 26. The first driven gear 34 of the output shaft 26further has a helix angle ψ_(out1), defined as the unsigned value of theangle formed between a tangent to the gear's helix at the pitch circle,and the direction of the central axis A_(out) of the first driven gear34, and the second driven gear 35 has a helix angle ψ_(out2) defined inaccordance with the same principles.

The drive gear 44 of the input shaft 28 is a helical gear of a firsthand, said first hand in this example being right hand, defined as theteeth twisting clockwise as they recede from an observer looking alongthe axis A_(in) of the drive gear 44 of the input shaft 28. The drivegear 44 of the input shaft 28 further has a helix angle ψ_(in), definedas the unsigned value of the angle formed between a tangent to thegear's helix at the pitch circle, and the direction of the central axisA_(in) of the drive gear 44. The same definition of helix angle applies,mutatis mutandis, to the other helical gears of the gear unit 24.

When the output shaft 26 is, via the layshaft 30, rotated in the drivingdirection 19 (FIG. 1), said driving direction being counter-clockwise,as seen in the direction of the central axis A_(in) of the input shaft28 towards the output shaft 26, the helical first driven gear 34, or thehelical second driven gear 35, as the case may be, which are axiallyfixed to the output shaft 26, will generate an axial thrust F1 acting onthe output shaft 26. Due to the hand of the driven gears 34 and 35 ofthe output shaft 26, the axial thrust F1 will be directed away from theinput shaft 28.

The drive gear 44 of the input shaft 28 will rotate in an inputdirection 47 which is the same as the driving direction 19, and willgenerate an axial thrust F2. The drive gear 44 of the input shaft 28 isaxially fixed to the input shaft 28, such that the axial force F2 willact on the input shaft 28. Due to the hand of the drive gear 44 of theinput shaft 28, the axial thrust F2 will be directed away from theoutput shaft 26. The directions of the axial thrusts F1, F2 areillustrated by arrows. According to an alternative embodiment a gearunit could be designed with the axial thrusts F1, F2 directed towardseach other.

As shown in FIG. 9 b, the output shaft 26 has an inner portion 25 oflower diameter than an outer portion 27. Hence, the output shaft 26 is atapering shaft. The second driven gear 35, forming part of the secondgear of the gear unit 24, transmits a lower torque than the first drivengear 34. Hence, a lower diameter of inner portion 25 is sufficient forthe torques transmitted by the second driving gear 35, which savesweight. A notch 31 at the transition from the inner portion 25 to theouter portion 27 serves as an axial direction support against which thesecond driven gear 35 may rest.

The output shaft 26 and the input shaft 28 are concentric, and meet at amain thrust bearing support 150, as best shown in FIG. 9 b. The inputshaft 28 is journalled to the output shaft 26 via a main thrust bearingarrangement 156 arranged in a support wall 55 of a gear unit housing 45,which allows the output and input shafts 26, 28 to rotate independentlyof each other. The support wall 55 is located between the axial endwalls 78, 80 of the housing 45. The layshaft 30 extends through anopening 57 in the support wall 55. Optionally, if radial support isrequired, a bearing may be arranged in the opening 57 for supporting thelayshaft 30. The fact that the main thrust bearing arrangement 156 isarranged the support wall 55 provides radial support to the bearingarrangement 156. The main thrust bearing arrangement 156 of FIG. 9 b issimilar to the arrangement 156 described hereinbefore with reference toFIG. 6 b and comprises a first tapered roller bearing 158 and a secondtapered roller bearing 160, which are arranged in an axial recess 174 ofthe output shaft 26. The two tapered roller bearings 158, 160 taper inopposite directions, such that they together form a bidirectional thrustbearing arrangement. Thereby, the axial thrusts F1, F2 of the output andinput shafts 26, 28 at least partly cancel each other out at the mainthrust bearing arrangement 156 irrespective of the input direction ofthe input shaft 28. The main thrust bearing arrangement 156 therebyforms an axially rigid support structure for non-yieldingly receivingaxial loads from both the output shaft 26 and the input shaft 28.

The output shaft 26 is journalled in an auxiliary bearing 176, which ismounted on a bearing support 150. The auxiliary bearing 176 does notneed to be a thrust bearing, since axial thrust will mainly be borne bythe main thrust bearing arrangement 156. Similar to what has beendescribed above with reference to FIG. 5 a-b, however, the auxiliarybearing 176 may, as an alternative, be a thrust bearing that can be usedfor preloading the output shaft 26 against e.g. a second auxiliarybearing 68. Also the input shaft 28 may be preloaded in a preloadingarrangement formed by an auxiliary axial thrust bearing 70 and the mainthrust bearing arrangement 156.

Neither the bearing support 150 nor the auxiliary bearing 176 mountedthereto are necessary for balancing the dynamic axial thrusts that occurwhen operating the gear unit 24; hence, they can be dispensed with, andradial support may be provided by other means available to the personskilled in the art.

When torque is supplied to the input shaft 28 in the input direction 47,the axial thrusts F1, F2 cancel out as a tensile force where the outputand input shafts 26, 28 meet in the main thrust bearing support 150.During normal operation of a vehicle 10 (FIG. 1), the average andinstantaneous torque supplied to the mobile vehicle gear unit 24 istypically larger in the input direction 47, i.e. forward movement of thevehicle, than in the opposite direction. The opposite direction may, forexample, be used when the vehicle 10 (FIG. 1) is to be reversed.Reversing of an electric drive vehicle may be made by simply shiftingthe rotational direction of the electric motor, to a direction which isopposite to the input direction 47. When the vehicle is reversed theaxial thrusts F1, F2 cancel out as a compression force where the outputand input shafts 26, 28 meet in the main thrust bearing support 150. Theconditions described hereinbefore with reference to expressions (1)-(3)may be applied also to the output and input shafts 26, 28 of FIGS. 9 a-bto obtain the best possible cancelling out of axial thrusts F1 and F2.

In the embodiment of FIGS. 9 a-b the output shaft 26 and the input shaft28 are concentric, and meet at a main thrust bearing arrangement 156being similar to the main thrust bearing arrangement 156 describedhereinbefore with reference to FIG. 6 b. It will be appreciated thatother types of bearing arrangements may also be combined with the mobilevehicle gear unit 24 of FIGS. 9 a-b. For example, the output shaft 26and the input shaft 28 of the gear unit 24 of FIGS. 9 a-b could bearranged in an output shaft main thrust bearing arrangement 56 and aninput shaft main thrust bearing arrangement 62 of the type describedhereinbefore with reference to FIG. 5 b, or in a main thrust bearingarrangement 156 of the type described hereinbefore with reference toFIG. 7 b.

In order to achieve a minimum total axial load onto the gear unithousing 45, also the layshaft 30 may be axially balanced. This may beachieved by the layshaft 30 being equipped with a respective driven gear38 and respective drive gears 36, 37 of the same hand, preferably with alarger helix angle on its respective larger pitch diameter gear than onits respective smaller pitch diameter gear. In the exemplary mobilevehicle gear unit 24 of FIG. 9 b, the first drive gear 36 of thelayshaft 30 has a first drive gear helix angle ψ_(drive,G1) the seconddrive gear 37 has a second drive gear helix angle ψ_(driveG2), and thedriven gear 38 has a driven gear helix angle ψ_(driven) of the same handas the first and second drive gears 36, 37. Thereby, when the outputshaft 26 is rotated in said driving direction 19, at least a portion ofthe axial thrusts generated by the helical gears 36, 37, 38 of thelayshaft 30 will cancel out as a compression force in the layshaft 30.It will be appreciated that when the gear unit 24 is set in first gear,the forces of the first drive gear 36 should cancel out, at leastpartly, the axial thrust of the driven gear 38, and when the gear unit24 is set in second gear, the forces of the second drive gear 37 shouldcancel out, at least partly, the axial thrust of the driven gear 38.

The pitch diameter D_(drive,G1) of the first drive gear 36 is smallerthan the pitch diameter D_(driven) of the driven gear 38; hence, thedriven gear helix angle ψ_(driven) is preferably larger than the firstdrive gear helix angle ψ_(drive,G1) such that axial balance is stillfurther improved. A similar reasoning can be made with respect to thepitch diameter and gear helix angle of the second drive gear 37. Theconditions described hereinbefore with reference to expressions (4)-(6)may be applied also for balancing axial thrusts of the layshaft 30 ofFIG. 9 b. The layshaft 30 is journalled in bearings 71, 73 in therespective end walls 78, 80 of the housing 45. The bearings 71, 73 needonly, when the axial thrusts of the layshaft 30 are at least partlycancelled out, as described hereinbefore, take up radial forces, andrather small axial thrusts.

In all embodiments described hereinbefore, the at least partialcancelling out of axial forces F1, F2 in an axially rigid supportstructure results in reduced movement of at least the output and inputshafts 26, 28 and their associated gears 34, 44. This allows for atighter backlash compared to what is possible to obtain in gear units ofprior art. In other words, the mobile vehicle gear unit 24 may bedesigned as a low-backlash gear unit. As a rule of thumb, the angulargear play θ_(G1) of a gear G₁ having n_(G1) teeth, the gear G₁ matingwith another gear G_(G2), which is held fixed, may be obtained throughthe relation

θ_(G1)=tan⁻¹(k/n _(G1))   (7)

wherein the number k is determinative of the angular play. As applied toan exemplary gear engagement of the mobile vehicle gear unit 24described hereinbefore, the angular gear play θ₃₄ of the driven gear 34of the output shaft 26, having n₃₄ teeth, may be obtained through therelation

θ₃₄=tan⁻¹(k/n ₃₄)   (8)

assuming that the drive gear 36 of the first layshaft 30 is heldimmobile. Typically, a suitable angular gear play of each gear 34, 36,38, 40, 42, 44, relative to its respective mating gear, is obtained fork<0.1. Hence, for a reasonably tight low-backlash gear unit, themajority, and preferably all of the gears 34, 36, 38, 40, 42, 44 of thegear unit 24 have an angular gear play θ_(G) fulfilling the relation

θ_(G)<tan⁻¹(0.1/n)   (9)

wherein n is the number of teeth of the respective gear. An even tighterlow-backlash gear unit may be obtained provided that the majority, andpreferably all of the gears 34, 36, 38, 40, 42, 44 of the gear unit 24have an angular gear θ_(G) play fulfilling the relation

θ_(G)<tan⁻¹(0.07/n)   (10).

An axial play allowing a pair of shafts S₁, S₂, engaging via helicalgears G₁, G₂, to move relative to each other results in an angular playbetween the shafts, since the relative axial translation of the shaftsS₁, S₂ will make the gears G₁, G₂ turn in their helical engagement.Hence, a reduced axial movement of the shafts in a gear unit alsodirectly results in a reduced angular play between the shafts.

For the pair of shafts S₁, S₂, the same rule of thumb may be applicableprovided that the gears G₁, G₂ are axially fixed to their respectiveshaft S₁, S₂; the angular play θ_(S1) of the shaft S₁ relative to theshaft S₂ may be obtained through the relation

θ_(S1)=tan⁻¹(k/n _(G1))   (11)

wherein the gear G₁ has n_(G1) teeth, the number k again beingdeterminative of the angular play.

For a gear unit 24 designed for at least partly balancing axial thrusts,and hence reducing axial movement, according to the guidelines disclosedherein, the angular shaft play θ_(S) of each shaft 26, 28, 30, 32, asconnected to another shaft via mating gears, preferably corresponds to anumber k<0.11, more preferably to a number k<0.08, and even morepreferably to a number k<0.07. Thereby, the total angular shaft playbetween the input and output shafts 26, 28 will be low.

Although possible in theory, it is in practice, due to e.g. friction,oil viscosity, production tolerances, wear etc., impossible to perfectlycancel out the axial forces F1, F2 to exactly 100%. Therefore it ispreferred that both output and input shafts 26, 28, as well as theirrespective gears 34, 44, be axially fixed relative to the housing 45,e.g. by means of a thrust bearing arrangement. Thereby, they will nottranslate axially, as the load conditions change, while the gear unit 24is operated. This is of particular value in a gear unit for the varyingload conditions typical of a mobile vehicle gear unit 24, sincesignificant axial translation may cause shafts or gears to reach an endposition in the gear unit housing 45, resulting in damage to the gearunit 24. Axial translation of the output or input shafts 26, 28 may alsodamage any upstream or downstream equipment such as the motor 16 or thedrive shaft 20.

Even though not necessary, the output and input shafts 26, 28illustrated in the examples hereinbefore are also radially fixedrelative to the gear unit housing 45. Thereby, they will not translateradially due to changing load conditions while the mobile vehicle gearunit 24 is operated.

In order to at least partly balance axial forces within each layshaft30, 32, the respective driven and drive gears 38, 42, 36, 40 of eachlayshaft 30, 32 are axially fixed relative to each other. By way ofexample, the drive gear 36 of the first layshaft 30 is axially fixedrelative to the driven gear 38 of the same layshaft 30. Also thelayshafts 30, 32 may be axially fixed relative to the gear unit housing45. The layshafts 30, 32 may also be preloaded between respective pairsof axial preload bearings (not shown) in a manner similar to thepreloading of the output and input shafts 26, 28.

The invention has mainly been described above with reference to a fewembodiments. However, as is readily appreciated by a person skilled inthe art, other embodiments than the ones disclosed above are equallypossible within the scope of the invention, as defined by the appendedpatent claims.

For example, it will be appreciated that features of the differentembodiments disclosed hereinbefore may be combined, so as to form stillfurther embodiments. By way of example, the expressions (1)-(6)disclosed with reference to FIG. 4 represent preferred relations betweentransmission ratio, gear pitch diameters, and respective helix anglesvalid for all embodiments.

Moreover, depending on the desired transmission ratio, the gear unit maybe provided with any number of layshafts connected in series between theinput shaft and the output shaft, for example a single layshaft or threelayshafts. In order to obtain the axial thrust balance describedhereinbefore, the respective hands, pitch diameters and helix angles ofthe driven gear of the output shaft and the drive gear of the inputshaft should be selected accordingly, as has been describedhereinbefore.

The gear unit may also be provided with any number of layshaftarrangements connected in parallel between the input and output shafts.The mathematical expressions and conditions above relating to axialbalance are still valid, if the parallel layshaft arrangements aresimilar with respect to helix angles and gear radii. For non-similarparallel layshaft arrangements, the expressions (1)-(6) may need to beadjusted accordingly, as will be appreciated by those skilled in theart.

It is not necessary that the mobile vehicle gear unit 24 be placed in ahousing so as to form a separate gearbox; alternatively, the gear unit24 may be built into the motor 16, into a common structure in whichshafts may be journalled, and to which a main thrust bearing support 50may be attached.

The gear unit 24 may form a part of a larger gear unit or system; i.e.,the gear unit 24 may be combined with other gearing, connected to theinput and/or output shafts 26, 28, so as to form a larger gear system.By way of example, the gear unit 24 may be connected to a planetarygear, which, together with the gear unit 24, forms a gear system havinga total transmission ratio different from that of the gear unit 24alone.

It has been described above how the mobile vehicle gear unit 24 can beused for providing a transmission ratio within a vehicle 10. However,the field of application for a gear unit according to the invention isnot limited to vehicles 10, such as cars, busses and lorries; by way ofexample, mobile, axial thrust-balanced gear units may as well be used inother mobile applications, e.g. ships for sea-transportation, forexample passenger transportation ships and cargo-ships, in which casethe gear unit may be installed for stepping down the rpm of a dieselengine to a suitable rpm for the propeller or other suitable propellingdevice. Another example of a mobile vehicle application is aircraft, inwhich the mobile, axial thrust-balanced gear unit may be used forstepping down the rpm of the turbine or engine driving the aircraft.

The terms “helical gear” and “helix angle” are to be interpretedbroadly, so as to include gears having teeth that are curved, such asspiral gears, but as a whole follow a generally helical path, such thatthose gears function in a manner equivalent to helical gears.

1. A method of stepping down the speed of a rotary motion from a firstspeed, supplied to an input shaft, to a second speed delivered by anoutput shaft, comprising transmitting said rotary motion to said outputshaft, via a driven gear of said output shaft, the driven gear beinghelical and having a first helix angle, from a layshaft arrangement, soas to generate a first axial thrust of the output shaft in a first axialdirection; transmitting said rotary motion to said layshaft arrangement,via a drive gear of said input shaft said drive gear being helical andhaving a second helix angle that is larger than said first helix angle,from said input shaft, so as to generate a second axial thrust of theinput shaft in a second direction, said second direction beingsubstantially opposite to said first direction; and applying at least aportion of said first axial thrust and at least a portion of said secondaxial thrust to the same location of an axially rigid support structure,such that said first and second axial thrusts counter-act and at leastpartly cancel out in said support structure.
 2. The method according toclaim 1, said axially rigid support structure being a thrust bearingarrangement interconnecting said output and input shafts.
 3. The methodaccording to claim 1, said axially rigid support structure being a mainthrust bearing support axially supporting said output shaft and saidinput shaft.
 4. The method according to claim 1, further comprisingtransmitting said rotary motion via at least one layshaft of saidlayshaft arrangement; and for each layshaft of said layshaftarrangement, generating a drive gear axial thrust in a helical drivegear having a drive gear helix angle; generating a driven gear axialthrust in a helical driven gear having a driven gear helix angle, saiddriven gear helix angle being larger than said drive gear helix angle;directing the drive gear axial thrust in said second direction; anddirecting the driven gear axial thrust in said first direction, suchthat the axial thrust of the respective drive and driven gears of eachlayshaft at least partly cancel out within said layshaft.
 5. Mobilevehicle gear unit comprising an output shaft, and an input shaftsubstantially parallel with said output shaft, the gear unit beingconfigured for providing a transmission ratio between said input shaftand said output shaft via a layshaft arrangement, the output shaft beingprovided with a driven gear in mesh with a drive gear of said layshaftarrangement, and the input shaft being provided with a drive gear inmesh with a driven gear of said layshaft arrangement, the transmissionratio being non-unity such that one gear of the driven gear of theoutput shaft and the drive gear of the input shaft will be arranged foroperating at a relatively higher torque, and the other gear will bearranged for operating at a relatively lower torque, said relativelyhigher torque being higher than said relatively lower torque, whereinthe output shaft being journalled in an output shaft main thrust bearingarrangement, mounted to a main thrust bearing support, and beingarranged for limiting axial movement of the output shaft in a firstaxial direction; the input shaft being journalled in an input shaft mainthrust bearing arrangement, said input shaft main thrust bearingarrangement being co-located with said output shaft main thrust bearingarrangement on said main thrust bearing support, said input shaft mainthrust bearing arrangement being arranged for limiting axial movement ofthe input shaft in a second axial direction, said second axial directionbeing substantially opposite to said first axial direction; said mainthrust bearing support rigidly connecting the output shaft main thrustbearing arrangement to the input shaft main thrust bearing arrangement;said driven gear of said output shaft being helical of a first hand;said drive gear of said input shaft being helical of a second hand, saidsecond hand being the same as the first hand for a positive transmissionratio and opposite to said first hand for a negative transmission ratio;and said gear arranged for operating at a relatively lower torque havinga helix angle exceeding the helix angle of said gear arranged foroperating at a relatively higher torque.
 6. Mobile vehicle gear unitaccording to claim 5, said main thrust bearing support being fixed to agear unit housing.
 7. Mobile vehicle gear unit according to claim 5,said output shaft main thrust bearing arrangement being arranged on afirst side of said main thrust bearing support, and said input shaftmain thrust bearing arrangement being arranged on a second side of saidmain thrust bearing support, said second side being opposite to saidfirst side.
 8. Mobile vehicle gear unit comprising an output shaft, andan input shaft substantially parallel with said output shaft, the gearunit being configured for providing a transmission ratio between saidinput shaft and said output shaft via a layshaft arrangement, the outputshaft being provided with a driven gear in mesh with a drive gear ofsaid layshaft arrangement, and the input shaft -being provided with adrive gear in mesh with a driven gear of said layshaft arrangement, thetransmission ratio being non-unity such that one gear of the driven gearof the output shaft and the drive gear of the input shaft will bearranged for operating at a relatively lower torque, and the other gearwill be arranged for operating at a relatively higher torque, saidrelatively higher torque being higher than said relatively lower torque,wherein the output shaft being journalled to the input shaft in a mainthrust bearing arrangement arranged for limiting axial movement of theoutput shaft relative to the input shaft in a first axial direction;said driven gear of said output shaft being helical of a first hand;said drive gear of said input shaft being helical of a second hand, saidsecond hand being the same as the first hand for a positive transmissionratio and opposite to said first hand for a negative transmission ratio;and said gear arranged for operating at a relatively lower torque havinga helix angle exceeding the helix angle of said gear arranged foroperating at a relatively higher torque.
 9. Mobile vehicle gear unitaccording to claim 8, said output shaft being arranged on a first sideof said main thrust bearing arrangement, and said input shaft beingarranged on a second side of said main thrust bearing arrangement, saidsecond side being opposite to said first side.
 10. The mobile vehiclegear unit according to claim 5, said main thrust bearing arrangement orarrangements being bidirectional for limiting axial movement of therespective output and/or input shaft in two axial directions.
 11. Themobile vehicle gear unit according to claim 5, each of said output andinput shafts being axially preloaded in a preloading arrangement. 12.The mobile vehicle gear unit according to claim 5, said output shaft andsaid input shaft being substantially concentric.
 13. The mobile vehiclegear unit according claim 5, said driven gear of said output shafthaving an output shaft driven gear pitch diameter D_(out) and an outputshaft driven gear helix angle ψ_(out); said drive gear of said inputshaft having an input shaft drive gear pitch diameter D_(in) and aninput shaft drive gear helix angle ψ_(in); and said driven gear of saidoutput shaft and said drive gear of said input shaft satisfying acondition corresponding to:${0.2 < {{I_{tot}\frac{D_{in}\tan \; \Psi_{out}}{D_{out}\tan \; \Psi_{in}}}} < 5},$wherein I_(tot) is the transmission ratio of said gear unit.
 14. Themobile vehicle gear unit according to claim 5, said layshaft arrangementcomprising at least one layshaft and optionally a plurality ofsubstantially parallel layshafts connected in series, each layshaftbeing provided with a helical drive gear and a helical driven gear, thedriven gear of each layshaft being of the same hand as the drive gear ofthe same layshaft.
 15. The mobile vehicle gear unit according to claim14, wherein, for each layshaft i of said at least one layshaft, or saidplurality of layshafts, the respective drive gear has a drive gear pitchdiameter D_(drive, i) and a drive gear helix angle ψ_(drive, i); therespective driven gear has a driven gear pitch diameter D_(driven, i)and a driven gear helix angle ψ_(driven, i), the driven gear helix angleψ_(driven, i) being different from the drive gear helix angleψ_(drive, i); and0.2<|(D _(drive, i)*tan ψ_(driven, i))/(D _(driven, i)*tanψ_(drive, i))|<5.
 16. The mobile vehicle gear unit according claim 5,wherein said gear unit is a step-down gear unit.
 17. The mobile vehiclegear unit according to claim 5, said gear unit having a transmissionratio, between said input shaft and said output shaft, of less than30:1.
 18. The mobile vehicle gear unit according to claim 5, said gearunit having a variable transmission ratio.
 19. The mobile vehicle gearunit according to claim 5, said gear unit being a gear unit for anelectric drive vehicle.
 20. The mobile vehicle gear unit according toclaim 5, wherein the driven gear of the output shaft is axially fixed tothe output shaft, and the drive gear of the input shaft is axially fixedto the input shaft.
 21. The mobile vehicle gear unit according to claim5, wherein for each layshaft of said layshaft arrangement, the drivengear of the layshaft is axially fixed relative to the drive gear of thelayshaft.
 22. The mobile vehicle gear unit according to claim 5,wherein, for each shaft of said gear unit, the angular shaft play θ_(S)relative to another shaft of said gear unit satisfies the conditionθ_(S)<tan⁻¹(0.11/n _(G)) wherein n_(G) represents the number of teeth ofa gear of said shaft, said gear being in engagement with a gear of saidanother shaft.
 23. The mobile vehicle gear unit according to claim 5,wherein the main thrust bearing support is a support wall locatedbetween the axial end walls of the gear housing, the output shaft 26 istapering outwardly
 24. The mobile vehicle gear unit according to claim5, wherein the output shaft is tapering inwardly in a direction toward amain thrust bearing arrangement.